1. Field of the Invention
The present invention relates to a control method and a combustion control system for operating a waste incineration plant and to a waste incineration plant.
Waste incineration plants serve, in general, for the compacting, hygienic treatment and mineralization of waste and for the concentration and inertization of pollutants contained in the waste, along with emissions which are preferably environmentally compatible. The energy occurring during waste incineration and the residues occurring at the end of incineration may be delivered for multilayered utilization or exploitation. Thus, for example, the calorific value of the waste may be utilized, by means of steam generation, for the provision of process steam or distant heat and for generating electric energy. The slag residue occurring at the end of the incineration process may be used as a useful material, for example in roadbuilding.
2. Discussion of Background Information
A control method for operating a waste incineration plant is described, for example, in EP0499976. In this method, a uniform heat or steam generation is achieved via a multiloop control device in which the generated steam power (steam quantity per unit time) is conducted as the main controlled variable and the oxygen content in the flue gas is conducted as an auxiliary controlled variable. For this purpose, a combustion control system capable of carrying out the method is equipped with a measurement device for the steam quantity and the oxygen content of the flue gas and with a low-speed main controller for the steam power. A high-speed O2 controller is connected in parallel with the main controller, and its outputs are connected to the ram, grate and primary-air flaps as actuators.
To minimize the environmental pollution caused by exhaust gases from a waste incineration plant, statutory regulations stipulate increasingly that a minimum firing temperature, as it is known, should not be undershot, since only by adhering to this minimum firing temperature is an incomplete combustion of the exhaust gases avoided and, for example, the fraction of organic substances in the exhaust gases limited. To ensure the minimum firing temperature which may be set, for example, at 850° C., auxiliary burners are used in modern waste incineration plants. Auxiliary burners, by burning fossil fuels, for example oil or gas, make it possible to keep the firing temperature above the corresponding minimum value.
When auxiliary burners are used, in particular, two aspects must be borne in mind:    1. A burner steam power as a proportion of the steam power is generated as a function of the burner power of the auxiliary burner and influences the overall combustion control system which normally manages without the ignition of auxiliary burners; and    2. on account of the uniform quality of the fuels for the auxiliary burner and because the burner power is therefore easy to control, conventional combustion systems tend, with a decreasing oxygen content in the flue gas, to brake the incineration of waste and thereby tend to replace the steam power from the waste incineration by a burner steam power generated by the auxiliary burner.
The result of this is that, in existing combustion control systems, a change often has to be made to a hand-controlled operating mode, and the return from operation with a auxiliary burner to normal waste incineration operation is delayed for an unnecessarily long time and increased operating costs, along with reduced waste throughput, are thereby incurred.